Various techniques have been proposed to create realistic three-dimensional (3D) images, as perceived by a viewer. In general, most of these techniques involve using an image projector to project stereoscopic images onto a remote display screen. For example, original movie content originating from two cinema cameras separated by a small angle can be used to generate the stereo effect. When a viewer processes the images from each camera with a different eye, the viewer interprets these two related viewpoints as a 3D image of the corresponding scene. As is well known in the art, the 3D image corresponding to the two viewpoints can be recognized when the viewer views the image with 3D glasses.
In active 3D image technology, the glasses are typically powered and include moving or switchable elements. Unfortunately, these systems can be very expensive. In addition, since the glasses must be accurately synchronized with the projector that sends the images to the display, it can be challenging to achieve the necessary synchronization between the glasses and the signal source.
In passive 3D image technology, the viewer typically wears low-cost eyeglasses that do not have any active or powered components, but rather provide some optical effect that can discriminate between the two images projected on the screen. For example, anaglyph glasses, which include the well-known red-cyan glasses, can be used to discriminate images made up of two color layers, superimposed, but offset with respect to each other. Unfortunately, this technology has severe limitations, including poor color fidelity due to the use of tinted glasses. Accordingly, there has been continuing interest in polarization-based passive 3D image technology.
Traditionally, polarization-based 3D image technology has required two projectors, each of which is used to superimpose a different viewpoint on the display screen. More specifically, each projector includes or is coupled to a polarizer/retarder such that images corresponding to the two different viewpoints are projected with different polarization states. For example, in one embodiment each projector includes a linear polarizer such that the two viewpoints are projected with light having orthogonal polarization states. More commonly, each projector includes a circular polarizer so that the two viewpoints are projected with left and right-circularly polarized light (e.g., circular polarization technology has the advantage over linear polarization methods in that viewers are able to tilt their head and look about naturally without disturbing the 3D perception). In each case, the display screen is typically treated to retain the polarization state (e.g., is a polarization maintaining screen, which for example, is coated with silver or aluminum). The corresponding passive 3D glasses typically include two polarizing filters, each of which only allows light having a certain polarization therethrough (e.g., one lens is a circular polarizer allowing right-circularly polarized light to pass while the other lens is a circular polarizer allowing left-circularly polarized light to pass). Unfortunately, this projection system is also costly, as it typically requires two projectors. Moreover, if two projectors are used, they must be aligned very accurately such that the two images are correctly aligned on the display screen.
Another polarization-based 3D image technology that has recently gained more attention uses a single projector and includes a push-pull electro-optical liquid crystal modulator that is disposed in front of the projector lens. The left and right perspective fields of the push-pull modulator alternately produced left and right (or right and left) handed circularly polarized light at a sufficiently rapid rate to produce a generally flicker free stereoscopic effect for the viewer. For example, some examples of push-pull electro-optical liquid crystal modulators for use in projection systems are discussed in U.S. Pat. Nos. 4,792,850 and 7,477,206. While the cost of these systems is reduced due to the use of a single projector, the use of the push-pull modulator has been associated with inadequate image clarity, low dynamic range, slow transition time, poor transmission characteristics, and other performance issues, particularly when used in the theatrical environment.
In US Pat. Appl. No. 200502374487, a single projector is used in combination with a color wheel assembly to provide the stereoscopic imaging. The color wheel assembly includes a first portion able to polarize light in a first orientation and second portion able to polarize light in a second orientation. The color wheel is rotated through the different color and polarization orientations to provide the stereoscopic images. While the cost of this imaging system is also reduced relative to the dual projector system, the success of this system is expected to be limited by the rotation of the two polarizing portions of the color wheel. In particular, it is noted that as each polarizing portion is rotated about the common rotation axis, the uniform fast axis orientation of each polarizer will change with time for a particular illumination field. More specifically, light incident on a predetermined spot on the color wheel will be transmitted with a time varying polarization, including polarizations that are not one of the desired first and second orientations discussed above.